Generally, tracking algorithms for the satellite communication is classified into a closed loop method and an open loop method. The closed loop method is further classified into a lobing method and a mono-pulse method.
The closed loop method is designed to control the antenna in a predicted orbit direction by processing satellite orbit forecasting data, standard time data, and antenna digital angle data using a computer. Therefore, the tracking performance of the antenna depends on the accuracy of the data. The lobbing method is designed to control the orientation of the antenna by detecting a coming direction of a bicorn wave by moving a beam of the antenna using a predetermined method. The mono-pulse method is designed to detect an azimuth error on occasion in accordance with a radio wave with a single pulse in a state where the beam of the antenna is fixed.
The lobbing methods are further classified into a conical scanning method, a beam switching method, and a step tracking method. The conical scanning method is designed to rotate the beam of an antenna in a conical-shape having a minute angle to perform a closed tracking. The beam switching method is designed to determine a relative receiving signal level while discretely moving the beam to more than four pre-determined locations disposed around the axis of the antenna. The step tracking method is designed to move the beam in a direction where the receiving level is increased by checking the variation of the receiving level while moving the antenna by a minute angle in a step manner at a predetermined time interval.
FIGS. 1 and 2 show a conical scanning antenna having an offset dual reflector structure in accordance with the related art. In the concrete, a conventional method for implementing conical scanning using the rotation of a sub-reflector is shown.
FIG. 1 shows the central axis C of a sub-reflector 101 disposed to be deviated from the central axis C of the main-reflector 100. FIG. 2 shows the sub-reflector 101 tilted from the central axis C at a predetermined angle.
As shown in FIGS. 1 and 2, in order to implement the conical scanning using the rotation of the sub-reflector in a dual reflector antenna structure, the sub-reflector must have a circular shape with an axial symmetry characteristic.
However, offset dual reflector antenna systems generally employ a sub-reflector having a predetermined asymmetric shape to have an axial asymmetric characteristic in order to optimize the performance thereof. Therefore, the tracking method using the rotation of the sub-reflector may cause a tracking beam to have an asymmetry characteristic for an asymmetric axis.